Use opensim-org's fork of BTK rather than klshrinidhi's
Use opensim-org's fork of BTK rather than klshrinidhi's

Fixes #1788.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core